Introduction
Human consciousness flows continuously and spontaneously from one thought to the next. Sometimes thinking evolves into an endless stream of repetitive self-referential negative thoughts that are difficult to let go of (Tseng & Poppenk, 2020), defined as rumination. According to the Response Styles Theory, rumination is the focused attention on the symptoms of one’s emotional distress and its possible causes and consequences, as opposed to its solutions (Nolen-Hoeksema, 1991; Nolen-Hoeksema et al., 2008). The habitual tendency to ruminate is considered a maladaptive form of emotional regulation and has been associated with a sustained physiological stress response that damages the hormonal, cardiovascular, and nervous systems, ultimately leading to observable physical and mental health problems (Aldao et al., 2010; Brosschot et al., 2006; Kubzansky et al., 1997). Rumination is one of the reasons responsible for incomplete cardiovascular recovery after stress exposure and is therefore associated with cardiovascular indices (Radstaak et al., 2011). Besides, even though the tendency to ruminate is a well-known predictor of the onset and maintenance of depression (McLaughlin et al., 2007; Nolen-Hoeksema et al., 2008), rumination is considered a more general transdiagnostic factor of vulnerability and outcome in stress-related psychopathology (e.g., alcohol abuse, eating disorder, anxiety; Caselli et al., 2010; Dondzilo et al., 2016; Wilkinson et al., 2013). Therefore, core neurophysiological mechanisms underlying a tendency to ruminate are important to further understand the relationship between rumination and the development of future clinical symptoms and health problems.
The functional state of the Autonomic Nervous System (ANS), consisting of the sympathetic and parasympathetic systems, has been reported as an important neurophysiological mechanism associated with rumination (Ottaviani et al., 2016; Vanderhasselt & Ottaviani, 2021). More specifically, both branches of the ANS act on the sino-atrial node of the heart and result in a complex variation between consecutive heartbeats over time, defined as heart rate variability (HRV; Goldberger et al., 2001; Porges, 2001, 2007; Saul et al., 1990; Task Force, 1996). In the face of stressors, the inhibitory control of the parasympathetic (over the sympathetic) nervous system enables the regulation of physiological and psychological states, an ability that is considered key to behavioral adaptability to the environment (mainly via the vagus nerve; Thayer et al., 2012). This parasympathetic dominance is associated with higher HRV (Friedman, 2007; Grol & De Raedt, 2020; Nasso et al., 2019, 2020; Pulopulos et al., 2018). Higher HRV is regarded as an indicator of better emotional regulation and mental health (Brosschot et al., 2006; Perna et al., 2020; Reynard et al., 2011; Thayer & Lane, 2000). In addition, the heart and the brain are reciprocally connected via the ANS pathways. Thayer’s neurovisceral integration model proposes a network of cortical and subcortical neural structures, known as the central autonomic network (CAN), which receives sensory input from the peripheral end organs such as the heart (Thayer & Lane, 2000). The neurovisceral integration model proposes that vagally-mediated HRV is a read-out of the bi-directional interaction between central and cardiovascular processes, with higher HRV denoting more self-regulation and adaptability (Thayer et al., 2009).
Even though the association between rumination and vagally-mediated HRV has been widely studied, research remains inconsistent across the literature. Rumination can be separated into trait and state components, with the habitual tendency to ruminate in daily life referring to trait rumination, whereas state rumination is the act of ruminating elicited by discrete stressors (Key et al., 2008; Nolen-Hoeksema & Morrow, 1993). Most often, dynamic changes in HRV have been reported to examine the link between state rumination and stress reactivity after a laboratory induction (Ottaviani & Shapiro, 2011). In these studies, where individuals were put into a state of rumination in the laboratory - such as a video clip triggering stress, coping with fear sources that frighten the individual, or engaging in fearful or angry imagery or recall - a reduction of HRV was observed (Beauchaine et al., 2007; Castaneda & Segerstrom, 2004; El-Sheikh et al., 2011). Furthermore, in a meta-analysis reviewing 18 experimental studies, it was reported that perseverative cognition, including rumination and worry that share a commonality and are highly intercorrelated (Brosschot et al., 2006b; Fresco et al., 2002), was associated with lower HRV in healthy volunteers only on the state measures (Ottaviani et al., 2016). However, in contrast to the inverse association between stress-induced HRV and rumination response, questions remain regarding the association between resting (baseline) HRV and the tendency to ruminate in daily life (trait rumination). The habitual tendency to ruminate is known to be associated with chronic stress (inability to recover) and has been associated with a higher risk of cardiovascular disease (Busch et al., 2017; Schwartz et al., 2003). Importantly, the meta-analysis mentioned above reported that there was no association between HRV and the trait measure of perseverative cognition in the experimental study; however, the number of correlational studies (and the number of subjects) testing this association was relatively low (Ottaviani et al., 2016). Therefore, the relationship between resting HRV and trait rumination in healthy individuals remains unclear, as many studies focus on clinical populations or the HRV in response to induced rumination (i.e., a momentary, reactive state). Even though resting HRV is assumed to be a transdiagnostic and promising biomarker of mental health resilience (Beauchaine & Thayer, 2015; Perna et al., 2020), to the best of our knowledge, there is no study describing the association between resting-state HRV and trait rumination based on a large-scale and well-powered dataset in healthy individuals.
In this study, we use a large cross-sectional dataset of physiological and self-report (trait rumination and depression) data from our laboratory collected over a time span of 4 years (2017-2022) to investigate the association between resting HRV (i.e., measured during a baseline period) and the habitual tendency to ruminate in healthy individuals. We hypothesize that baseline HRV at rest will be negatively correlated with trait rumination as both high trait rumination and low HRV are risk factors for stress-related psychopathology (Key et al., 2008; Larsen & Christenfeld, 2009). In addition, given that HRV is sensitive to various demographic variables, we will investigate whether HRV is associated with the tendency to ruminate after controlling for gender, age, and the level of depressive symptoms that are known to significantly influence HRV (Laborde et al., 2017). Moreover, a study has indicated that parasympathetic regulation of the heart through the vagus nerve might not have an unmitigated linear relationship with emotion (Kogan et al., 2013), so we also explore whether there is a non-linear relationship between HRV and emotion in our large cross-sectional data.